Sensitivity is one of the most important characteristics of a receiver - the ability to detect the presence (or absence) of a signal.
Selectivity is the ability to detect a signal in the presence of other signals that are not on the frequency you want to receive. Indeed, if a strong signal adjacent to you is getting through, you wouldn't hear the one you're trying to receive, but let's ignore that, since it's the wrong answer.
Total Harmonic Distortion (THD) is a popular term that describes how well an amplifier reproduces its input. It's usually applied to high-end audio amplifiers. In this case, however, they're asking about detecting a signal, not how well an amplifier reproduces a signal.
And linearity refers to getting a proportional increase or decrease when you vary something, like the gain of an amplifier - this is a topic covered on the General exam, but not on the Technician exam. Besides, linearity isn't about detecting, but about how well the output of a circuit matches the shape of the input to the circuit.
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A transmitter is used to convert information to radio waves, and a receiver is used to convert radio waves back into information. Your radio in your car used to listen to music is an example of a receiver, where the station broadcasting the music has a transmitter that is tuned to the same frequency your car radio is.
A transceiver is ideal for 2-way communication because it facilitates both the talking and listening aspects of communication. Transceivers are the more common in ham radio than dedicated transmitters, although many hams have receive-only sets as well.
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A mixer allows two signals to be combined such that there will be an output signal at the sum and difference of the two signals.
For instance, if you mix a 14.250 MHz signal with a 2.4 kHz signal, there will be an output at 14.2524 MHz and 14.2476 MHz.
Sum of 14.250 MHz + 2.4 kHz
Difference of 14.250 MHz - 2.4 kHz
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Selectivity is the ability of a receiver to discriminate a desired signal from other adjacent signals. If the IF (Intermediate Frequency) circuits do not filter out adjacent signals then they will also be detected and interfere with the desired signal. To help learn this remember: We need very good Selectivity in order to Discriminate the signal properly.
The distractor answers may need some explanation so you know why they are not the correct answer:
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Oscillators generate a signal of a desired frequency.
Reactance modulators are circuits that are used to add voice information to radio signals - they don't generate the signal.
A product detector is a circuit in a receiver - it doesn't generate a radio signal.
And a low pass filter keeps radio frequency energy above a certain cut-off level from passing through it, but it doesn't generate RF energy, so it doesn't generate any sort of signal.
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This question wants to know what electrical circuit converts a lower frequency Single-Side-Band (SSB) frequency to a higher frequency. This is done by a Transverter. The other choices will not convert one frequency to another.
More information can be found here: Transverter
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PTT means Push To Talk.
If you've never heard of or seen a Push To Talk (PTT) switch on the side of a microphone, you might be forgiven for guessing that PTT means something about pre-transmission, or precise tone, or primary transformer tuner... but it doesn't, it just means Push To Talk.
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A Modulator is the circuit that "modulates" or encodes the RF carrier frequency with the speech signal in a transmitter. The keyword "combines" indicates that the voice and carrier are put together, which should help you envision a circuit that modulates the signal.
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Weak-signal communication involves radio signals whose strength levels are close to or even partially embedded in natural noise. This is a tricky thing, and it generally can't be done with a normal FM transceiver. Different types of antennas could be used, but most importantly a multi-mode transceiver is needed so that modes with lower bandwidth and signal requirements can be used.
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This one is probably pretty obvious if you think about it; an RF power amplifier amplifies (or increases) an RF signal from another device.
Note that a voltage regulator (the most common incorrect response to this question) only regulates the voltage, and does not directly affect power. A voltage regulator might be used as part of a power supply to keep the output voltage constant.
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A preamplifier is used to amplify a signal before it gets to the receiver; this might be used if the receiver is not sensitive enough, but most commonly a preamplifier is installed as close to the antenna as possible and before the feedline in order to compensate for feedline loss.
Preamplification is only done on a received signal and is "pre" because it occurs before the receiver receives the signal.
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